Test dependabot for Dockerfiles & Bump Postgres to 11 (#557)

* Test dependabot for Dockerfiles.

* Also for github actions

* Update dependabot.yml

* Fix urls.py for new versions of Django

* Fix urls.py for postorius.

* Bump dependency on Postgresql.

* Bump dependency on Django

* Add dependency on tzdata.

docker-compose.yaml

@@ -49,7 +49,7 @@ services:
     - POSTGRES_DB=mailmandb
     - POSTGRES_USER=mailman
     - POSTGRES_PASSWORD=mailmanpass
-    image: postgres:9.6-alpine
+    image: postgres:11-alpine
     volumes:
     - /opt/mailman/database:/var/lib/postgresql/data
     networks:
@@ -62,4 +62,4 @@ networks:
       driver: default
       config:
       -
-        subnet: 172.19.199.0/24
+        subnet: 172.19.199.0/24